A number of vanadium catalysts have been described in the literature based on species such as VCl.sub.4, VCl.sub.3, VOCl.sub.3, and vanadyl tris-acetylacetonate. Impregnated on or chemically anchored to supports such as silica, these catalysts provide polyethylene with a broad molecular weight distribution and certain desirable properties. However, these catalyst systems are plagued by several problems, among them, low activity. Particularly in a gas phase process, high activity and hence low catalyst residues, obviating the need for removal, are desired.
Recently vanadium catalysts have come to the fore in polyethylene production because of the discovery of a particular catalyst composition, which is singularly effective in gas phase processes. The advantages of this catalyst composition are superior hydrogen response thus achieving very low levels of unsaturation in the polymer; a high degree of productivity resulting in very low catalyst residues; and the production of polyethylenes having a relatively broad molecular weight distribution and a wide melt index range. Further, they permit the utilization of a broad range of alpha-olefin comonomers, which enables the production of a wide range cf densities; and they provide polymers having high bulk densities and readily fluidizable particle sizes low in fines, substantially free of agqlomeration.
The particular catalyst composition referred to comprises (i) the reaction product of a vanadium trihalide and an electron donor and a modifier impregnated onto a silica support; (ii) a halocarbon promoter; and (iii) a hydrocarbyl aluminum cocatalyst. The catalyst composition can be prepared as follows: the vanadium trihalide is first dissolved in the electron donor at a temperature in the range of about 20.degree. C. to below the boiling point of the electron. donor for about 2 to about 6 hours. Silica is then added as a dry powder or as a slurry in the electron donor or an inert solvent whereupon the silica becomes impreqnated with the vanadium trihalide/electron donor reaction product. The liquid is then removed by drying at less than 100.degree. C. for about 3 to about 6 hours. The modifier, dissolved in an inert solvent, is subsequently mixed with the impregnated carrier, and the liquid is removed by drying at a temperature of less than about 70.degree. C. for about 2 to about 4 hours. The halocarbon promoter and the hydrocarbyl aluminum cocatalyst are added to the supported catalyst either before and/or during polymerization.
Since its introduction into commerce, the art has sought to optimize this catalyst composition particularly with regard to its polymerization activity.